1. Field of the Invention
The present invention relates to a control device for vehicle brake systems.
2. Discussion of the Related Art
Heretofore, as one form of control devices for vehicle brake systems, there has been known one which is described in Japanese unexamined, published patent application No. 11-20670 (JP11-20670 A equivalent of (U.S. Pat. No. 6,322,164 B1). As shown in FIG. 11 of the Japanese published application (FIG. 101 of the U.S. patent), the control device of a vehicle brake system takes thereinto a booster pressure signal (step S5) when a booster pressure switch 204 is normal in operation, and then, judges based on the taken booster pressure signal whether the booster 12 is in the state of a boosting limit reached or not (step S6). If judging that the booster 12 is in the state of the boosting limit reached, the control device performs a pressure increase control (step S8). Concretely, the control device activates a pump 62 to generate in brake cylinders 50 a pressure which is higher by a differential pressure ΔP than a master cylinder hydraulic pressure PM so that the braking effect can be made to be stable irrespective of the booster 12 being in around the boosting limit (refer to FIGS. 13 and 14 of the Japanese published application (FIGS. 103 and 65 of the U.S. patent). On the other hand, if judging that the booster is not in the state of the boosting limit reached, the control device performs a processing for terminating the pressure increase control (step S7).
However, regarding the booster 12 in the foregoing brake system, the booster 12, due to its structure, brings about a fluctuation (i.e., temporary drop) of the vacuum pressure in its vacuum pressure chamber when a brake pedal 10 is returned. Specifically, as shown in FIG. 7 of the Japanese published application, when the brake pedal 10 returns, a control valve 23a takes seat on an air valve 23b as it is separated from a vacuum valve 23c. This causes a variable pressure chamber 18 to be blocked from the atmosphere and to communicate with a vacuum pressure chamber 17, whereby the pressure of the variable pressure chamber 18 drops temporarily. As a consequence, the pressure in the vacuum pressure chamber 17 in communication with the variable pressure chamber 18 also drops temporarily. In this way, there is arisen a problem that the variation in the vacuum pressure makes it impossible to terminate the pressure increase control at an appropriate timing.